1. Field of the Invention
The present invention relates to a surface acoustic wave device and a method for producing the same.
2. Description of the Related Art
A surface acoustic wave device is used, for example, in a high frequency circuit of cellular phones. FIG. 12 shows a SAW filter 1 in a plan view as an example of a conventional surface acoustic wave device (hereinafter, referred to also as a SAW device). The SAW filter 1 includes a piezoelectric substrate 2 and a pair of interdigital transducers 3a and 3b (hereinafter, referred to also as IDT electrodes) formed on the substrate 2. The substrate 2 is formed of LiTaO3, LiNbO3 or the like.
In the SAW filter 1, if the substrate 2 has a pyroelectric property, a change in temperature in an assembly process or a change in temperature when used as a device causes non-uniformity in electric charge distribution on the surface of the substrate 2. As a result, a potential difference is generated between the IDT electrodes 3a and 3b so that discharge occurs between the electrodes. The discharge between the electrodes causes melting and cutting-off of the electrodes, resulting in deterioration or shift of the characteristics of the filter.
In order to solve the above problem, the following three methods have been proposed. A first method is to connect the two IDT electrodes by a fine metal electrode (see JP 3-29407A). However, since this metal electrode is fine, the electric connection is easily broken, for example by heat. Thus, this method cannot provide a reliable filter.
A second method is to form a thin film having a predetermined resistivity that covers the IDT electrodes (see JP1-106611A, JP10-303681A, and JP11-74750A). However, in the above method, since the IDT electrodes are covered with the thin film, the characteristics for propagation of surface acoustic waves may be deteriorated. In addition, a process for forming the thin film is required.
A third method is to treat the substrate with heat in a reducing gas to reduce the surface resistance, and then to form IDT electrodes (see JP11-92147A)
However, the SAW filter obtained by the third method has a problem in that insertion loss of high frequency characteristics is increased. It is believed that this is caused by lattice defects generated significantly deep in the substrate when the substrate is treated with heat in a reducing gas, which affects the propagation of the surface acoustic wave energy.